Hillary Clinton has expanded her lead over Sen. Bernie Sanders, I-Vermont, in the key early voting state of Iowa, according to a poll released Monday by The Des Moines Register/Bloomberg Politics.

The survey found 48 percent of likely Democratic caucus goers said Clinton is their first choice for president, up from 42 percent in early October. Thirty-nine percent, meanwhile, said Sanders is their first choice for president, up from 37 percent in October. Four percent said they support former Maryland Gov. Martin O'Malley.

Older Democrats in Iowa tend to support Clinton while young voters prefer Sanders, the latest poll found. Nearly two-thirds of people 65 and older said they back Clinton while 58 percent of people under the age of 45 said they prefer Sanders.

Clinton has a 19-percentage-point lead over Sanders when it comes to women voters and leads Sanders 55 to 30 percent among people who earn $100,000 or more a year.

The poll, however, found Sanders leads Clinton 48 to 44 percent among self-identified liberal and 51 percent to 26 percent among independent voters.